Model and report changes

1. The model now accounts for the ongoing immunisation programme, stratifying the population of people still susceptible to infection with the virus according to their immunisation status (unimmunised/1 dose/2 doses). We use data on the daily proportions of the population getting immunised to inform this splitting of the population, assuming that it takes three weeks for vaccine-derived immunity to develop. Vaccine efficacy is assumed against both infection and death, using values for the efficacy in agreement with those found here. We have a changepoint in the vaccine efficacy on the 10th May, which marks a transition from alpha being the dominant variant, to delta.
2. The model now also accounts for a different susceptibility to infection in each adult age group (no prior information is used); and for the under-15s, (using prior information from Viner et al, 2020, which estimates children to be less likely to acquire infection when in contact with an infectious individual).
3. The model has the ability to incorporate estimates of community prevalence, by region and age group, from the Office of National Statistics COVID-19 Infection Survey (see Data Sources for details). These are included weekly since the outset of the Survey in May 2020 for the age groups >4 years to inform trends in incidence that are too recent to be captured by the data on deaths.
4. The geographical definition has been changed from the seven NHS regions (map) to the nine regions typically used in government (map). This new spatial definition more appropriately reflects the existing regional heterogeneity.
5. The underlying probability of an unvaccinated individual dying following infection with SARS-CoV2 (the infection-fatality rate, IFR) is allowed to change gradually over the course of 30 days every (approximately) 100 days. This is designed to reflect fluctuations due to seasonal effects, demand on healthcare services or the emergence of new virus variants of differing severity.
6. The ‘Epidemic summary’ now only reports the current value for the IFR by age. To visualise how this has changed over time in our model, see the IFR tab in the ‘Infections and Deaths’ section of the report. The quantity that is now plotted under this tab is the probability of dying if infected, taking into account the impact of the immunisation programme.

Updated findings

1. The estimate of the daily number of new infections on the 30th July across England is 54,400 (45,600–64,700, 95% credible interval). The daily infection rate is estimated to be the highest in the North East (NE) with 162 infections per 100K population per day. This corresponds to 4,300 new daily infections. The next highest rates of infection are in the West Midlands (WM) and the Yorkshire and Humber (YH) with 154 and 106 infections per 100K (9,080 and 5,830 daily infections), respectively. London (GL) and the South East (SE) have around 90 new infections per 100k each day, followed by the East of England (EE) and South West (SW) with daily rates of about 70 per 100k. The North West (NW) is the region with the lowest incidence rate with 39 infections per 100K. Note that a substantial proportion of these daily infections will be asymptomatic.
2. In contrast to the number of infections, the number of deaths occurring daily has increased and, for the 20th August we forecast between 51 and 102 deaths.
3. The probability of Rt exceeding 1 has decreased in all regions. It is highest at 54% in the SE, close to 25% in four regions and is only 7% in GL, 3% in the SW and negligible or zero in the NW and NE.
4. The growth rate for England has decreased to -0.01 (-0.02–0.00) per day. This means that, nationally, the number of infections is highly likely to be decreasing, and this pattern is common to the majority of regions.
5. GL, followed by the NE and the WM, have the highest attack rates, that is the proportions of the regional populations who have ever been infected, with 37%, 36% and 33% respectively. The SW continues to have the lowest attack rate at 18%. These attack rates are entirely consistent with our previous published report.
6. Note that the deaths data used are only very weakly informative on Rt over the last two weeks and are thankfully sparse. Therefore, the estimate for current incidence, Rt and the forecast of daily numbers of deaths are likely to be subject to some revision.

Interpretation

The plots of the estimated Rt over the most recent weeks show the Rt following a declining trend from peaks of around 1.50 to values just below 1. This downward trend in Rt from the peak is still largely driven by the indices of mobility, and particularly by the recent school holiday.

The incidence of deaths, which has been falling since the end of March, has increased again over the past six weeks and continues to rise, although the actual numbers remain low in comparison to the two previous waves of infection. Our projections for the number of deaths are close to a peak by the end of the three-week period.

Plots of the IFR over time show that from the end of January we estimate a decreasing IFR in all adult age groups, but most steeply in the older ages. This drop indicates the benefits of immunisation against death over and above the benefits against infection. Specifically, there is an estimated fall to a still-high 2.5% (2.3%–2.8%) in the over-75s and 0.15% (0.14%–0.16%) overall. The overall impact of the immunisation programme can be seen more clearly in the ‘All Ages’ plot, where the precipitous decline in IFR since late January is a product of this efficacy against death but also of the increasing proportion of infections in young people; older age groups are immunised and become more protected against infection. The impact of the immunisation in the 25–44 age-group is beginning to become apparent with a fall after an initial plateau.

For context, alongside the data used here, reported new positive tests have suggested a declining epidemic over the last couple of weeks. This trend is highly dependent on the propensity to test and, therefore, difficult to interpret. However, an initial downturn has also started to appear in hospital admissions in some regions. Prevalence of infection, as estimated by the ONS Coronavirus Infections Survey, remains high at close to 1.60% in England, but the rate of increase is slowing down.